From the very beginning of the oil and gas well drilling industry, as we know it, one of the biggest challenges has been the fact that it is impossible to actually see what is going on downhole. There are any number of downhole conditions and/or occurrences which can be of great importance in determining how to proceed with the operation. It goes without saying that all methods for attempting to assay such downhole conditions and/or occurrences are indirect. To that extent, they are all less than ideal, and there is a constant effort in the industry to develop simpler and/or more accurate methods.
In general, the approach of the art has been to focus on a particular downhole condition or occurrence and develop a way of assaying that particular thing. For example, U.S. Pat. No. 5,305,836, discloses a method whereby the wear of a bit currently in use can be electronically modeled, based on the lithology of the hole being drilled by that bit. This helps the operator know when it is time to replace the bit.
The process of determining what type of bit to use in a given part of a given formation has, traditionally, been, at best, based only on very broad, general considerations, and at worst, more a matter of art and guess work than of science.
Other examples could be given for other kinds of conditions and/or occurrences.
Furthermore, there are still other conditions and/or occurrences which would be helpful to know. However, because they are less necessary, and in view of the priority of developing better ways of assaying those things which are more important, little or no attention has been given to methods of assaying these other conditions.
Surprisingly, to applicant""s knowledge, no significant attention has been given to a method for assaying the work a bit does in drilling a hole from an initial point to a terminal point. The present invention provides a very pragmatic method of doing so. The particular method of the present invention is relatively easy to implement, and perhaps more importantly, the work assay provides a common ground for developing assays of many other conditions and occurrences.
More specifically, a hole is drilled with a bit of the size and design in question from an initial point to a terminal point. As used herein, xe2x80x9cinitial pointxe2x80x9d need not (but can) represent the point at which the bit is first put to work in the hole. Likewise, the xe2x80x9cterminal pointxe2x80x9d need not (but can) represent the point at which the bit is pulled and replaced. The initial and terminal points can be any two points between which the bit in question drills, and between which the data necessary for the subsequent steps can be generated.
In any event, the distance between the initial and terminal points is recorded and divided into a number of, preferably small, increments. A plurality of electrical incremental actual force signals, each corresponding to the force of the bit over a respective increment of the distance between the initial and terminal points, are generated. A plurality of electrical incremental distances signals, each corresponding to the length of the increment for a respective one of the incremental actual force signals, are also generated. The incremental actual force signals and the incremental distance signals are processed by a computer to produce a value corresponding to the total work done by the bit in drilling from the initial point to the terminal point.
In preferred embodiments of the invention, the work assay may then be used to develop an assay of the mechanical efficiency of the bit as well as a continuous rated work relationship between work and wear for the bit size and design in question. These, in turn, can be used to develop a number of other things.
For example, the rated work relationship includes a maximum-wear-maximum-work point, sometimes referred to herein as the xe2x80x9cwork rating,xe2x80x9d which represents the total amount of work the bit can do before it is worn to the point where it is no longer realistically useful. This work rating, and the relationship of which it is a part, can be used, along with the efficiency assay, in a process of determining whether a bit of the size and design in question can drill a given interval of formation. Other bit designs can be similarly evaluated, whereafter an educated, scientific choice can be made as to which bit or series of bits should be used to drill that interval.
Another preferred embodiment of the invention using the rated work relationship includes a determination of the abrasivity of the rock drilled in a given section of a hole. This, in turn, can be used to refine some of the other conditions assayed in accord with various aspects of the present invention, such as the bit selection process referred to above.
The rated work relationship can also be used to remotely model wear of a bit in current use in a hole, and the determination of abrasivity can be used to refine this modeling if the interval the bit is drilling is believed, e.g. due to experiences with nearby xe2x80x9coffset wells,xe2x80x9d to contain relatively abrasive rock.
According to another embodiment of the present invention, work of the bit can be determined using bit mechanical efficiency, where the mechanical efficiency of the bit is based upon a percentage of a total torque applied by the bit which is cutting torque. As a result, effects of the operating torque of a drilling rig or apparatus, being used or considered for use in a particular drilling operation, on mechanical efficiency are then taken into account with respect to assaying the work of the bit. The present invention thus includes a bit work analysis method and apparatus, including a method for modeling bit mechanical efficiency, are disclosed herein below. The present invention is also implementable in the form of a computer program.